So if you sign into Gmail first with test@example.com and then with notatest@gmail.com, https://mail.google.com/mail/u/0/ will go to the inbox of test@example.com; but if you sign into notatest@gmail.com before test@example.com, the same link will go to the inbox of notatest@gmail.com.

Aside from always signing into Google accounts in the same order, is there a good way to link to a specific Gmail account?

A related question that I didn't specifically ask: how to permalink to a specific message ID of a specific mailbox... but that's a different question :)
– DanApr 26 '14 at 0:42

Related: Google Voice link to a specific account: https://www.google.com/voice/?authuser=test@example.com. This only works correctly if you are already logged into the specified account though.
– Jordan BroughAug 10 '14 at 12:40